Transmissometer

ABSTRACT

An improved transmissometer for determination of the density or opacity of a gaseous sample. A modulated light beam is passed through a gaseous sample. A retroreflector positioned on the far side of the sample returns the light beam through the same. A movable chopping disc sequentially and in a repetitive cycle moves three discrete segments into the path of the modulated light beam before the light beam enters the sample. The segments include a cut-out portion to allow passage of the light beam into the sample; a first reflector portion to reflect back substantially all of the modulated light beam; and a second reflector portion that reflects back a pre-selected percentage of the modulated light beam. A beam splitter, positioned between the modulated light beam source and the chopping disc, passes the light beam toward the disc, but reflects light returned through the sample and the chopping disc at an angle of about 90* to the path of travel of the beam. A main light sensing element receives light reflected by the beam splitter and generates an electrical signal in response thereto. Two independent sample and hold units respectively receive and present as a constant output the electrical value representing light reflected back through the sample and light reflected from the first reflector portion of the chopping disc. A sequencing system conditions the sample and hold units to accept a signal only when the chopping disc is properly oriented. A subtraction unit subtracts the value of the electrical signal representing light reflected through the sample from the electrical signal representing light reflected by the first reflector portion of the chopping disc.

United States Patent Ansevin et al.

[ TRANSMISSOMETER [75] Inventors: Ronald W. Ansevin; Melvin L.

Lowry, both of Pittsburgh; John E. Traina, Glenshaw, all of Pa. [73] Assignee: Contraves-Goerz Corporation,

Pittsburgh, Pa. [22] Filed: Oct. 21, 1974 [21] Appl. No.: 516,144

[52] US. Cl. 250/573; 250/343; 356/201 [51] Int. Cl. ..G01N 21/26 [58] Field of Search 250/573, 574, 339, 343; 356/201, 204, 207, 208

[56] References Cited UNITED STATES PATENTS 3,562,524 2/1971 Moore et al. 250/343 3,696,247 10/1972 Mclntosh et al 250/573 3,860,818 1/1975 Stadler et al 356/201 Primary Examiner-James W. Lawrence Assistant ExaminerD. C. Nelms Attorney, Agent, or FirmSteve M. McLary 57 ABSTRACT An improved transmissometer for determination of the density or opacity of a gaseous sample. A modulated light beam is passed through a gaseous sample. A

retroreflector positioned on the far side of the sample returns the light beam through the same. A movable chopping disc sequentially and in a repetitive cycle moves three discrete segments into the path of the modulated light beam before the light beam enters the sample. The segments include a cut-out portion to allow passage of the light beam into the sample; a first reflector portion to reflect back substantially all of the modulated light beam; and a second reflector portion that reflects back a pre-selected percentage of the modulated light beam. A beam splitter, positioned between the modulated light beam source and the chopping disc, passes the light beam toward the disc, but reflects light returned through the sample and the chopping disc at an angle of about 90 to the path of travel of the beam. A main light sensing element receives light reflected by the beam splitter and generates an electrical signal in response thereto. Two independent sample and hold units respectively receive and present as a constant output the electrical value representing light reflected back through the sample and light reflected from the first reflector portion of the chopping disc. A sequencing system conditions the sample and hold units to accept a signal only when the chopping disc is properly oriented. A subtraction unit subtracts the value of the electrical signal representing light reflected through the sample from the electrical signal representing light reflected by the first reflector portion of the chopping disc.

9 Claims, 5 Drawing Figures US. Patent Nov. 4, 1975 Sheet 2 of4 3,917,957

US. Patent Nov. 4, 1975 Sheet 3 of4 3,917,957

Sheet 4 of 4 3,917,957

U.S. Patent Nov. 4, 1975 m. q /l w wmJl TRANSMISSOMETER BACKGROUND OF THE INVENTION This invention generally relates to transmissometers of the type wherein the density or opacity of a gaseous sample is measured as a function of the attenuation of a light beam passed through the sample. More particu larly, this invention relates to such a device wherein a clear sample reference signal is generated every cycle to serve as a basis for comparison with the signal generated to represent the opacity of the sample under study. Yet more specifically, this invention relates to a device of the character described which further automatically generates a mid-range calibration signal on a periodic basis to give a running check on the satisfactory operation of the device.

The use of optical devices to measure the density or opacity of gaseous materials. smoke for example, is known in the art. Examples may be seen in US. Pat. Nos. 3,600,590; 3,617,756; and 3,810,697. Reference is also drawn to co-pending US. Pat. application Ser. No. 484,798, filed July 1. 1974 and having an assignee in common with the assignee of the present invention. The present invention allows calibration of the signal processing circuits each cycle through the use of automatic gain control function which compensates for component aging or dirt accumulation within the optical paths. In addition. in a cyclic pattern, a mid-range calibration check is made as well as a check of the calibration of the clear sample condition.

SUMMARY OF THE INVENTION Our invention is an improved transmissometer. In this general class of transmissometer, a light beam, modulated at a pre-selected frequency, is passed through a gaseous sample. The light beam is reflected back through the sample by a retro-reflector positioned across the sample from the source of the modulated beam. The components of the improved transmissometer include a main light sensing means, positioned out of the path of travel of the modulated light beam, which generates an electrical signal representative of the amount of light falling on it. Also included is a movable chopping means, which has at least three distinct seg ments, which sequentially and in a repeating cycle moves the three segments into the path of travel of the modulated light beam prior to passage through the sample. The three segments include a cut-out portion which allows passage of the modulated light beam through the sample; a first reflector portion which will reflect back substantially all of the modulated light beam; and a second reflector portion which will reflect back a preselected percentage of the modulated light beam. There is also a means, positioned in the path of travel of the modulated light beam, in advance of the chopping means, which will pass the light beam toward the chopping means but will direct any light from the direction of the chopping means onto the main light sensing means. A first electronic means receives and presents as a constant output signal for each cycle of the chopping means an electrical signal representative of the light reflected by the first reflector portion A second electronic means receives and presents as a constant output signal for each cycle of the chopping means an electrical signal representative of the light reflected by the retroreflector. An electronic subtraction means then subtracts the signal furnished by the second 2 electronic means from the signal furnished by the first electronic means. The final element is a sequencing means for generating an individual electrical signal indicating the presence at the main light sensing means of light reflected by the retro-reflector. the first reflector portion or the second reflector portion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the apparatus of the present invention;

FIG. 2 is a side elevational view partially in crosssection, of the main housing for the components of the present invention;

FIG. 3 is an elevational view taken along the line 33 of FIG. 2 showing the second chopping disc;

FIG. 4 is a view similar to FIG. 3 illustrating a slightly modified embodiment of the chopping disc of FIG. 3; and

FIG. 5 is a schematic circuit diagram of the elec tronic signal processing system of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS The overall operation of the transmissometer of the present invention is best understood with reference to the simplified block diagram of FIG. 1. In FIG. 1, a light source 10 furnishes a beam of radiant energy to a first chopping disc 12. The purpose of the first chopping disc 12 is to modulate the light furnished by the light source 10 at a desired frequency. This frequency may vary rather widely but is generally within the range of from 1,000 to 10,000 cycles per second. The modulated light beam from the first chopper 12 then passes through a beam splitter 14. The beam splitter 14 is of the conventional type which will pass a portion of radiant or light energy in one direction and will reflect a portion of the light returning from the opposite direction at an angle of approximately with respect to the original beam. In this case, the light from the first chopper 12 passes directly through the beam splitter 14 into a second chopping unit 16. The second chopping unit or chopper disc 16 has several unusual characteristics, which will be explained more fully later. For one part of the cycle of the second chopper 16, the light is passed through the second chopper l6 and through a samplle 18 whose characteristics are measured by this device. The sample 18 may be an enclosed space such as a smoke stack, thus allowing this unit to be directly mounted on the stack. Alternatively, however, the sam ple could be removed from a main stream of flowing material and sampled in a separate enclosed region if desired. The light passing through the sample 18 impinges upon a retro-reflector 20 positioned on the opposite side of the enclosed space through which the sample 18 passes and reflects back through the second chopper 16. The beam splitter 14 then directs a portion of the returning light downward at an angle of approxi mately 90 degrees to its original path of travel into a light-sensitive detector 22 which is a means for generat ing an electrical signal representative of the light falling on it. The detector 22 is the main sensing element of the entire system. The degree to which the light passing through the sample 18 and returning is attenuated is a measure of the opacity of the sample 18 under consideration. This opacity may be caused by a number of factors such as smoke comprised of very small particles or air-born particles of a fairly appreciable size. What ever is the cause of the opacity in the sample 18, this 3 instrument will give a relative measure of precisely how much opacity the sample 18 creates. Thus this device is useful as an air pollution monitoring device to ensure that the emissions from a smoke stack are within the requirements for such emissions. The modulated light beam could be directed across an open space with the retro-reflector 20 positioned across the space from the source of the beam. This configuration could be used. for example. to measure the general haze condition of the air in a particular locale. The signal generated by the detector 22 is transmitted through a conductor 24 to a current-to-voltage converter 26. The output of the detector 22 is a current and since most electronic elements today are designed to operate most conveniently with voltage signals. the current-to-voltage converter 26 is used to achieve this conversion from a signal current to a signal voltage. The voltage from the currentto-voltage converter 26 is transmitted along a conductor 28 to a narrow band pass filter 30. The filter 30 is a means for removing any signal components which are not within the frequency band which was determined by the first chopping unit 12. This helps to remove stray noise signals which may be introduced into the system. The output of the filter unit 30 is then transmitted through a conductor 32 to a demodulator unit 34. The demodulator 34 is a phase-sensitive unit which is a means for removing the frequency modulation introduced by the first chopping unit 12. To this end, a lightsensitive timing sensor 36 is associated with the first chopping unit 12. The timing sensor 36 transmits a signal along a conductor 38 to the demodulator 34. When the first chopping unit 12 is passing a light signal. the timing sensor 36 will pass the signal to the demodulator 34, and conversely, when light is not being passed by the first chopping unit 12 no signal will be present on the conductor 38. The net result is that the output of the demodulator 34 is a rectified DC level signal representing the varying signal which is actually presented by the main detector unit 22. The output of the demodulator 34 is transmitted along a conductor 40 to an amplifier 42. The output of the amplifier 42 is connected by a conductor 44 to a reference sample and hold unit 46. A branch conductor 48 connects the output of the amplifier 42 to a signal sample and hold unit 50. Both the sample and hold units 46 and 50 are designed to hold a signal passed to them for a period of time and present the signal as a constant output value for eventual measurement. The output of the reference sample and hold unit is connected through a conductor 52 to a difference amplifier S4. The output of the signal sample and hold unit is connected through a conductor 56 to the difference amplifier 54. This entire system is so constructed that at one point during each cycle a reference level is impressed into or on the reference sample and hold unit 46. This reference level indicates the level of light which should be returned should the sample 18 be perfectly clear or present no opacity or resistance to the return of light from the reflector 20. The signal sample and hold unit 50 accepts the actual signal which is passed through the sample 18 and returned to the detector 22. The difference amplifier 54 is designed to subtract these two values and present the output difference along a conductor 58 to a display unit 60. The number shown by the display unit 60 is a relative measure of the opacity of the sample under consideration. The display unit 60 may be a simple dial-type voltmeter. a digital voltmeter or may be a strip chart-type re corder to allow a permanent record to be made of the opacity of samples over a relatively long period of time. Depending upon the electronic signal components used. it may also be necessary to use a line driver 62 to boost the output signal along the conductor 58 to a proper level for utilization by the display unit 60.

Associated with the second chopping unit 16 are at least three read sensors generally designated as 64. The read sensors 64 are designed to present a signal which tells which of the various signals possible from the second chopping unit 16 back into the detector 22 through the beam splitter 14 are present at any one time. These signals are presented along output conductors 66, 67 and 68 from the read sensors 64. The conductor 66 carries information which indicates the presence of a 100% transmission signal on conductor 44. This is the signal which is present or would be present if the sample 18 presented no opacity to the passage of the light from the second chopping unit 16 to the retro-reflector 20 and back again. The conductor 66 is connected to a first switching unit 70 and is also connected to the reference sample and hold unit 46. At least once during each cycle of the second chopping unit 16 the reference l0()% transmission signal or reference signal is presented along a conductor 72 to the reference sample and hold unit 46 for storage and presentation to the difference amplifier 54. An automatic gain control (AGC) circuit is used to provide compensation to avoid signal changes that would result from aging of components. line voltage fluctuations, lamp aging. or coating of dirt or other substances from the environment in which the unit operates. This is an inevitable situation and in order to compensate for this situation, a comparator unit (or AGC difference amplifier) 74 is utilized. The output of the reference sample and hold unit 46 is connected to the comparator unit 74 through a conductor 76. A reference voltage designated as V R is furnished along a conductor 78 to the comparator unit 74. The reference voltage V is set at a known level which is the level at which [00% transmission would normally be occurring. If the output of the reference sample and hold unit 46 varies from this known reference voltage level, a corrective signal will be generated from the comparator unit 74 and will be transmitted along a conductor 80 to the amplifier 42 to modify the gain factor of the amplifier 42 in order to compensate for the previously described changes in the system. Under normal operational conditions. the conductor 68 from the read sensors 64, carries the informational signal to a third switch 82. The third switch 82 is connected to the signal sample and hold unit 50 by a conductor 84. This signal controls the signal and sample hold unit 50, which then transmits the signal to the difference amplifier 54 for subtraction with the signal from the reference sample and hold signal to give a measure of the actual opacity of the sample 18 under consideration. The conductor 67 carries a signal which is representative of a mid-scale or a partial scale calibration signal to a second switch 86. The second switch 86 is connected to the signal sample and hold unit 50 through a conductor 88. The output of the first switch 70 is also connected to the signal sample and hold unit 50 through a conductor 90 and a branch conductor 92 connects the first switch 70 to the difference amplifier 54. A timer 94 is connected to the switches 70, 86 and 82 through respective conductors 96, 97 and 98. The timer 94 controls the on and off cycling of these switches and thus determines which one of the various signals will be presented to the signal sample and hold unit 50 for comparison with the signal from the reference sample and hold unit 46. The use of a timer or timed interval is optional, and the various signals could be switched in and out manually whenever calibration check is desired. Under normal circumstances. the third switch 82 will be on for a relatively long period of time, for example, approximately one hour. The other two switches will be off during this period of time. Then, in order to obtain a calibration or a check of the system operation, the switches 70 and 86 will be alternately activated while the third switch 82 is held off in order to check both the 100% transmission level and the mid-span or partial span level as previously explained.

FIG 2 illustrates the mechanical and optical components of the transmissometer of the present invention. The entire unit is mounted in a housing or case 100. This case 100 may be hinged to provide easy access to the interior thereof for service or set-up of these components. The light source is mounted from a bracket 102 within the case 100. A lens system 104 transmits the light from the light source 10 to a mirror 106 which directs the light through the first chopper 12. The chopper 12 is driven in a continuously rotating fashion by a motor 108 mounted in the case 100. It should be noted that the first chopper 12 could conceivably be driven in an oscillatory fashion by a solenoid, vibrating reed, or similar device as opposed to being continuously rotated by the motor 108, but the continuous drive provided by the motor 108 has proven to be most convenient. The first chopper 12 is a generally discshaped member provided with a plurality of cut-outs in its periphery which thus modulates the light furnished from the light source 10 as alternate open and closed spaces pass between the mirror 106 and a second lens system 110 which further projects the light beam. The timing sensor 36 is made up of a light-emitting diode 112 and a light-responsive element 1 14 which are mounted on opposite sides of the chopper 12. These elements are so aligned that whenever a space in the chopping disc 12 is in alignment with the mirror 106 to allow passage of light, the light-emitting diode is also in alignment with a slot in the chopping disc 12 and thus allows the light-sensitive element 114 to generate a signal which is transmitted along the conductor 38 to the demodulator 34. The actual configuration of the device shown in FIG. 2 is somewhat different from that shown in the simplified block diagram of FIG. 1, and thus it is necessary to realize that more than one beam splitter such as that designated as 14 in FIG. 1 is used. Thus, instead of the initial light passing straight through the beam splitter 14, as is shown in FIG. 1, the light coming from the second lens 110 is actually bent at 90 degrees with respect to its original path of travel by the first beam splitter 14 and exits through a third lens system 116 and the second chopper 16. The second chopper 16 is driven by and continuously rotates due to a motor 118. The read sensors 64 are located near the lower most edge of the second chopper 16. These sensors are actually three in number and are pre-packaged units of a sideby-side light-emitting diode and light-responsive transistor which are angled such that light sent by the light-emitting diode may be reflected back to the lightresponsive transistor. These units are generally designated as 120, 121 and 122 and may be Texas Instrument Type TIL 139s. The case or housing 100 may be mounted to a smoke stack with the use of flange mounts 124 and the light itself is directed down a tubu- 6 lar portion 126 into the stack and across where it will be reflected by the retro-reflector 20 mounted on the opposite side.

FIG. 3 graphically illustrates how the rather specific configuration of the second chopper 16 aids in allowing calibration of this transmissometer to compensate for long-term drifts in equipment and for the eventual collection of dirt on the equipment which could lead to erroneous results. The second chopper 16 is a basically disc-shaped member which includes two open cut-out portions 128 and 130. When these portions are aligned with the beam splitter 14, light will pass through these portions. through the tubular portion 126, and across the sample 18 to be reflected back by the rctroreflector 20. Returning for a moment to FIG. 2, the return light beam will pass straight through the beam splitter 14 and will then impinge upon a second beam splitter 132. It is important to remember at this point that the second beam splitter 132 could be dispensed with and the main detector 22 could be placed in this position. However, for ease of setup of this device, the beam splitter 132 is employed and projects the majority of the light downward approximately degrees to its original path of travel into the main detector 22. An eye piece 134 is located in line with the return light beam and the second beam splitter 132 and allows visual observation of the alignment of the retro-reflector 20. With respect to FIG. 3 again, it should be realized at this time that the use of two segments 128 and for the projection of the light beam toward the retro-reflector 20 is simply for the purposes of allowing a higher rate of measu rement. It would be possible to have only a single segment that is open on the second chopper disc 16 and thus only one measurement would be made per rotation of the chopping disc 16. However, to allow a high rate of measurement and to keep down the speed of rotation of the chopping disc 16, it is preferred to use at least two segments so that two measurements may be made during each single rotation of the chopping disc 16. The two openings 128 and 130 each cover approximately 60 of the circumference of the circular disc 16. These sections are separated by approximately 180 degrees. Also on the chopping disc 16 are two areas or portions designated as 136 and 138 which are coated with a retro-reflective material and extend approximately 60 of the circumference of the chopping disc 16 and are separated by 180. These areas are designed to provide a 100% reflection reference signal. That is, the signal which would be generated by the light beam impinging upon these areas and being received by the main detector 22 would be equivalent to the signal re ceived if the sample 18 were completely clear and absorbed none of the light beam during the beams travel across and back through the sample 18. The chopping disc 16 is completed by two additional areas or portions designated as 140 and 142 which also occupy approximately 60 of their circumference of the disc 16 and are also coated with a retroreflective material. These areas 140 and 142 are selectively coated so that a light beam impinging upon them will reflect back to the main de tector 22 a signal representing approximately 50% absorption of the outgoing signal. These segments are provided to allow a mid-point span calibration signal. It would be possible to make these areas more or less reflective than 50%, but 50% provides a convenient point at which a calibration can be automatically made. The specific value of reflectance chosen can vary with each instrument. For example, the value for a short working distance would normally be 50%. Larger dimensions of working distances will use a reflectance value reduced in value to, for example, 30% in order to more nearly match the magnitude of an actual signal. Once set, this reflective value remains fixed for a given instrument. Located near the periphery of the second chopper 16, are two timing strips 144 and 146 made of a retroreflective material. The timing strips 144 and 146 are associated with the openings 128 and 130 and are located an equal radial distance outward from the center of the second chopping disc 16. The timing strips 144 and 146 are generally centered with respect to the openings 128 and 130 but are slighly smaller in angular extent, for example, being around 50 degrees as opposed to the 60 degree extent of the openings 128 and 130. This smaller angular extent is to make sure that the openings 128 and 130 are in complete alignment with the light beam before the measurement begins. The strips 144 and 146 are in alignment with the lowermost of the timing sensing units 122. For example, when the opening 128 is in alignment with the lens system 116 to allow light to pass outward into the sample, the timing strip 146 is in alignment with the read sensor package 122 which will cause generation of a signal as a result of the timing strip 146 reflecting light from the light-emitting diode to the photo transistor. Similarly, the 100% reflection areas 136 and 138 have timing strips 148 and 150 associated with them. The timing strips 148 and 150 are on a slightly smaller radius from the center of the chopping disc 16 to allow them to align with the read sensor package 121. Finally, the mid-span reflective areas 140 and 142 also have timing strips 152 and 154 associated with them. The timing strips 152 and 154 are on a still smaller radius to allow them to be in alignment with the read sensor package 120 to generate a signal when the mid-span area is to be read. The basic function served by the strips 144, 146, 148, 150, 152 and 154 and their associated read sensors is to provide a sequencing means for generating a signal that will indicate whether the light being received by the detector 22 is from the retro'reflector 20 or from one of the reflective areas 136, 138, 140 and 142 of the chopping disc 16.

FIG. 4 illustrates a slightly modified form of a chopping disc designated as 16A. In the chopping disc 16A, the open areas designated as 128A and 130A have been moved further outward toward the periphery of the disc 16A. This gives a somewhat larger time during which the light beam may pass through these openings since the openings themselves are larger in circumferential extent although their angular extent remains the same. Likewise provided on the discs 16A are 100% transmission areas 136A and 138A and span calibration areas 140A and 142A. The timing strips shown in FIG, 3 have been replaced in FIG. 4 by timing slots which now appear near the center of the disc 16A as opposed to being peripherally disposed on the disc 16. These slots are generally designated as 144A, 146A, 148A, 150A, 152A, and 154A. These slots serve the same function as the retro-reflective timing strips of FIG. 3. but, of course, would require that the lightemitting element be on one side of the chopping disc 16A and the light-responsive element be on the opposite side of the chopping disc 16A.

Another possible arrangement. not shown in the drawings, would be to mount the read sensors 120, 121 and 122 at 120 intervals around the chopping disc 16. In this case, only one or two, depending upon whether one or two open spaces are formed in a particular disc, timing slots or reflective areas would be required. The read sensor in alignment with the slot or reflective area would give a signal indicating which portion of the disc was in alignment with the sample. Placing the read sensors 120, 121 and 122 in this manner eliminates any cross-talk problems between sensors.

FIG. 5 shows the electronic circuit processing for this transmissometer in a schematic form. Some resistors and capacitors which are normally inserted for noise suppression or signal smoothing have been omitted in the interests of simplicity. It is also assumed that this circuit has a positive voltage supply designated as v-land a negative voltage supply designated as v. Also note that with respect to the read sensors 64, the read sensor packages 120, 121, and 122 are not necessarily stacked in the same configuration as that in which they are shown in FIG. 2. However, by following the numbering of their respective conductors, it is believed that no confusion should result from this. The light signal received by the main sensor 22 is carried by the conductor 24 to the current-to-voltage converter 26. The current-to-voltage converter 26 is a conventional design utilizing an operational amplifier 156 which has its negative input terminal connected to the conductor 24 and its positive input terminal grounded through a resistor 158. A feedback loop to the negative input terminal of the operational amplifier 156 from the output terminal thereof is completed through a feedback resistor 160. The output signal from the operational amplifier 156 is carried by the conductor 28 to the negative input terminal of an operational amplifier 162 which is the major component of the filter 30. A capacitor 164 and a resistor 166 are connected in series in the conductor 28 before the input to the negative input terminal of the operational amplifier 162. A positive input terminal of the operational amplifier 162 is connected to ground through a resistor 172. A capacitor 168 and a resistor 170 are connected in parallel in a feedback loop between the output terminal of the operational amplifier 162 and the negative input terminal of the operational amplifier 162. The major component of the demodulator 34 is an operational amplifier 174 having positive and negative input terminals and an output terminal. The conductor 32 connected to the output of the operational amplifier 162 is connected to the positive input terminal of the operational amplifier 174 through an input resistor 176. A grounded resistor 178 is connected to the resistor 176 and the positive input terminal of the operational amplifier 174 to provide a voltage dividing function. A feedback loop is formed between the negative input tenninal of the operational amplifier 174 and its output terminal with a feedback resistor 180 and a grounded voltage dividing resistor 182. The conductor 32 is also connected to the nega tive input tenninal of the operational amplifier 174 through the source and drain of a field effect transistor 184 and a series connected resistor 186. The gate of the field effect transistor 184 is connected by the conductor 38 to the photo transistor 114 which is a part of the timing sensor 36 for the first chopping disc 12. So long as the first chopping disc 12 is allowing light to pass, the photo transistor 114 will be fumishing a voltage signal to the gate of field effect transistor 184 to cause it to be on or conducting. During dark periods, the field effect transistor 184 will be off. The net result of the connections shown for the demodulator 34 is that previously described of converting the oscillating or modulated 9 light signal into a DC level signal. Note that both the light-emitting diode 112 and the photo transistor 114 are both connected to the positive voltage supply for the circuit, The conductor 40 is connected to the output of the operational amplifier 174 and is connected to the negative input terminal of an operational amplifier 188 for the amplifier 42 through an input resistor 190. The positive input terminal of the operational am plifier 188 is connected to ground through a resistor 192. A fixed resistor 194 and a photo-resistive element 196 are connected in a feedback loop between the negative input terminal of the operational amplifier 188 and the output terminal of the operational amplifier 188. The photo-resistive element 196 may be a device such as a cadmium sulphite cell whose resistance will vary as a function of the amount of light it is receiving. As will be explained shortly. this allows a variation in the total feedback resistance for the operational amplifier 188. Since the gain factor of an operational ampli fier is a function of both the input resistance and the feedback resistance, varying the feedback resistance will, in effect, vary the overall amplification factor of the total devicev The output terminal of the operational amplifier 188 is connected through the conductor 44 to the positive input terminal of an operational amplifier 198. The source and drain of a field effect transistor 200 are connected in the conductor 44 intermediate the operational amplifiers 188 and 198. The gate of the field effect transistor 200 is connected to the conductor 72 which in turn is connected to the conductor 66 from the read sensor package 121. A capacitor 102 is connected to ground and the positive input terminal of the operational amplifier 198. The negative input terminal of the operational amplifier 198 is tied to the output terminal of the operational amplifier 198. The components just described make up the reference sample and hold unit 46. The capacitor 102 will hold the value presented to it for a relatively long period of time with the operational amplifier 198 acting as an isolation device to prevent rapid discharge. Thus, the output of the operational amplifier 198 along the conductor 52 will be a value which is equivalent to the last value impressed upon the capacitor 102. Note that the conductor 72 which controls the operation of the gate of the field effect transistor 200 is connected in the conductor 66 prior to the first switch 70. Thus, whenever the read sensor package 121 generates a signal, which will occur whenever the segments 136 or 138 are in a position to reflect back the light from the light source to the main detector 22, the field effect transistor 200 will be on and the capacitor 102 will receive a value which is equivalent to this reflected light. This is the 100% transmission or zero opacity reference value for the sample 18. This number provides a reference source to allow comparison with a signal value which is received at times when the openings 128 and 130 are in alignment to allow light to pass through the sample 18. The output terminal of the operational amplifier 198 is also connected to the conductor 76 which itself is connected to the negative input terminal of an operational amplifier 204 through an input resistor 206. The operational amplifier 204 is the major component of the comparator unit or AGC difference amplifier 74. A feedback resistor 208 is connected between the nega tive input terminal and the output terminal of the operational amplifier 204. The reference voltage signal designated as V is connected to the positive input terminal of the operational amplifier 204 through an input 10 resistor 210. A grounded resistor 212 serves as a voltage divider to properly set the precise level for the reference voltage V So long as the output of the operational amplifier 198 is equal to the set reference voltage. the output of the operational amplifier 204 will be a fixed and known value. Thus, the output of the operational amplifier 204 controls the operation of a light emitting diode 214. So long as this value remains consistent, the light-emitting diode 214 will also give off the consistent quantity of light. The light-emitting diode 214 and the photo-resistive element 196 are preferably in one package so that the resistance of the photo-resistive element 196 is a direct function of the amount of light furnished by the light-emitting diode 214. Under ideal circumstances, the reflection from the areas 136 and 138 will give a consistent signal level which will remain at the reference voltage or V level. In this case, the value of the resistance of the photoresistive element 196 will not vary. However, as the various components of this system age and light outputs vary or a layer of dirt and grime begins to accumulate on some of the elements in the system, this output or reflection from the areas 136 and 138 on the chopping disc 16 may begin to vary. In this case, the output of the operational amplifier 198 will not necessarily be the V voltage. In this case, the output of the operational amplifier 204 will change which in turn will change the amount of light emitted by the light-emitting diode 214. This will also then have the effect of changing the effective resistance of the photo-resistive element 196. This in turn then will change the total gain factor of the operational amplifier 188 which will then have the effect of changing the output of the operational amplifier 198 back to its original value. The net result of this total system is that a consistent ratio will be maintained between the output of the signal sample and hold and the reference sample and hold 46 despite aging of components or collection of dirt on various elements. This, in effect, is an automatic gain control system. The signal sample and hold unit 50 is very similar inconcept and design to the reference sample and hold unit 46. The conductor 48 connects the output of the operational amplifier 188 to the positive input terminal of an operational amplifier 216. The negative input terminal of the operational amplifier 216 is connected to the output terminal of the operational amplifier 216. The source and drain of a field effect transistor 218 are con nected in the conductor 48 to the positive input terminal of the operational amplifier 216. A grounded eapacitor 220 is also connected to the positive input terminal of the operational amplifier 216. The gate of the field effect transistor 218 is connected to all three of the conductors 90, 88 and 84 from the three switches 70, 86 and 82. The field effect transistor 218 will be off so long as no signal is present at its gate, and therefore no signal will be presented to the operational amplifier 216 under these circumstances. The capacitor 220 will store whatever signal is presented to it from the operational amplifier 188. Then, when the field effect transistor 218 is oft", the operational amplifier 216 acts as an isolation device to prevent rapid draining of the capacitor 220 and will thus allow the presentation as a reasonably constant output on the conductor 56 the value held by the capacitor 220. Under most operating conditions, the signal which is desired to be held by the capacitor 220 is that which is generated by the passage of the light beam through the sample 18 and back from the retro-reflector 20 into the main detector 22. This 1 1 function is controlled by the third switch 82. Whenever the areas 145 or 144 are in alignment with the read sensor package 122, indicating that the slots or openings 128 or 130 are allowing passage of light through the sample 18, the third switch 82 will be on which will turn the field effect transistor 218 on, which in turn will allow the capacitor 220 to accept whatever value is generated by passage of the light beam through the sample 18 and back again. This value is the number of quantity which will give an indication of the total opacity of the sample 18 that is under consideration. Even though the field effect transistor 218 has been turned off by the removal of the opening 128 or 130 from a position in which the read sensor package 122 would turn the third switch 82 on. the capacitor 220 will continue to hold its previous value. The actual value desired is a percentage value or a percentage of clear stack transmission and the difference amplifier S4 is designed to provide this number. Remembering again that the conductor 52 carries a signal which is indicative of l% transmission or absolutely clear stack value, the conductor 52 is connected to the negative input terminal of an operational amplifier 222 through an input resistor 224. The output of the operational amplifier 216 is connected to the conductor 56 and to the positive input terminal of the operational amplifier 222 through a variable input resistor 226. The variable resistor 226 is used to revise the gain factors so that the actual voltages entering the difference amplifier may be fine tuned. A grounded resistor 228 is also connected to the positive input terminal of the operational amplifier 222 to complete this voltage-dividing function. This requirement is easily understood when one realizes that even in the case of an absolutely clear sample area there are some losses inherent in the transfer of the light across the sample area from the retro-reflector 20 and back to the main detector 22. However, these loss factors are much less in the case in which the light only has to travel the very short distance to the 100% reflection test areas 136 or 138 of the chopping disc 16. Thus, the clear stack signal which may be furnished by the operational amplifier 216 may actually be smaller than the signal furnished by the operational amplifier 198 even in the case of a completely clear sample. The resistors 226 and 228 compensate for this to allow an accurate comparison by the operational amplifier 222. A feedback resistor 230 is connected between the neg ative input terminal and the output tenninal of the operational amplifier 222. Thus, the output of the operational amplifier 222 carried along the conductor 58 is a value which is representative of the difference between the signal that would be present if the sample were completely clear and the actual signal received and caused by any opacity in the sample under measurement. The conductor 58 is connected to the negative input terminal of an operational amplifier 232 through an input resistor 234. The operational amplifier 234 is a major component of the line driver 62, and as was previously pointed out, may or may not be necessary dependent upon the particular type of output display device which is used. A feedback resistor 236 is connected to the negative input terminal of the operational amplifier 232 and to the output terminal of the opera tional amplifier 232. The positive input terminal of the operational amplifier 232 is connected to ground through a resistor 238. The output of the operational amplifier 232 is connected along a conductor 240 to the display device 60 which allows ready identification 12 or reading of the numerical difference between the clear sample situation and that presented by the actual sample under consideration.

The switches and 86 are used in various calibration modes to ensure that over a period of time the components of the system remain stable. The timer 94, as was previously noted, is designed to allow measurement of the actual sample 18 for a major portion of any time period. However, it is also set to periodically switch off the third switch 82, which will cause cessation of measurement of the sample 18, and sequentially switch on the first and second switches 70 and 86. When the first switch 70 is turned on by the timer 94, the read sensor package 121 will activate the field ef fect transistor 218 whenever the timing segments 148 and 150 are in front of it, thus allowing the signal for the transmission generated by the segments 136 and 138 to be fed to the operational amplifier 216 as well as to the operational amplifier 198. This allows a direct and simultaneous comparison of the same value from two different sources. Note that when this occurs, the signal also passes along the conductor 92 to a switching means 242. The switching means 242 is interposed intermediate a ground connection and a variable resistor 244 which is connected to the positive input terminal of the operational amplifier 222. This function is necessary because it will be recalled that the resistors 226 and 228 were used to compensate for the fact that even in a clear sample condition the value received by the operational amplifier 222 from the operational am plifier 216 would not necessarily be the same as that from the operational amplifier 198 due to losses in transmission of the light across the sample 18. However, under the conditions in which the first switch 70 has been activated, both the operational amplifiers 198 and 216 are receiving the signal from the segments 136 or 138. Thus, both signals should be the same and it is therefore necessary to compensate again to remove the previously introduced compensation factor from the output of the operational amplifier 216. One will be able to readily detect if there is any difference in the outputs of the operational amplifiers 198 and 216. There should not be any difference in these two values at this time. Likewise, when the second switch 86 is activated, keeping in mind that only one of the switches 70, 82 or 86 is activated any one time, the field effect transistor 218 will be turned on only when the read sensor package is in alignment with the timing strips 152 or 154 which indicate the presence of the areas 140 or 142 for span calibration. Under these circumstances, the value of the signal from the operational amplifier 216 should be one which would indicate midscale opacity of the sample 18. It should be kept in mind, of course, that at this time, the light beam passing through the sample area is not being read, but rather the areas 140 and 142 are being read as an indication of what the value would be should the sample itself absorb an intermediate amount of light presented to it. This again allows a calibration at some mid-range or intermediate value of the total range and allows a quick determination of whether or not the instrument is operating within its proper limits.

We claim:

1. An improved transmissometer of the type wherein a modulated light beam of a pre-selected frequency is passed through a gaseous sample, and wherein said light beam is reflected back through said sample by a retro-reflector positioned across said sample from the 13 source of said modulated light beam. said improved transmissometer comprising, in combination:

a main light-sensing means. positioned out of the path of travel of said modulated light beam, for generating an electrical signal representative of the amount of light falling thereon;

a movable chopping means, having at least three distinct segments, for sequentially and in repeating cycle, moving said three segments into the path of said modulated light beam, said segments including: a cut-out portion to allow passage of said modulated light beam through said sample; a first reflector portion to reflect back substantially all of said modulated light beam; and a second reflector portion to reflect back a preselected percentage of said modulated light beam;

means, positioned in the path of travel of said modulated light beam in advance of said chopping means, for passing said modulated light beam toward said chopping means and directing any light from said retro-reflector or said first or second reflector portions onto said main light-sensing means;

first electronic means for receiving and presenting as a constant output signal for each cycle of said chopping means, the electrical signal representative of the light reflected by said first reflector portion;

second electronic means for receiving and presenting as a constant output signal for each cycle of said chopping means, the electrical signal representative of the light reflected by said retro-reflector;

electronic subtraction means for subtracting said electrical signal representative of the light reflected by said retro-reflector from said electrical signal representative of the light reflected by said first reflector portion; and

sequencing means, connected to said first and second electronic means, for generating an individual electrical signal indicating the presence at said main light-sensing means of light reflected by said retroreflector, said first reflector portion or said second reflector portion.

2. The transmissometer of claim 1 which further includes:

timing means, connected to said second electronic means, for conditioning said second electronic means to receive the electrical signal representing the light reflected by said retro-reflector, said first reflector portion and said second reflector portion as individual, independent signals in a pre-selected, repeating time based cycle.

3. The transmissometer of claim 1 which further includes:

display means, connected to said electronic subtraction means, for visually displaying the difference signal generated by said subtraction means.

4. The transmissometer of claim 1 which further includes:

amplifier means, connected to said main light-sensing means and said first and second electronic means,

14 for amplifying the electrical signal from said main light-sensing means.

5. The transmissometer of claim 4 which further in cludes means, connected to the output of said first electronic means and to said amplifier means, for comparing the output signal value of said first electronic means with a pre-selected signal value and for generating a correctional signal to vary the gain factor of said amplifier means if said output signal of said first electronic means differs from said preselected signal value.

6. The transmissometer of claim 4 which further includes:

a currenbto-voltage converter, connected in circuit intermediate said amplifier means and said main light-sensing means, for converting an electrical current from said main light-sensing means to an equivalent electrical voltage.

7. The transmissometer of claim 6 which further includes:

electronic filter means, connected in circuit intermediate said current-to-voltage converter and said amplifying means, for removing all components of the electrical signal furnished by said main lightsensing means which are not at the frequency of modulation of said modulated light beam.

8. The transmissometer of claim 7 which further includes:

electronic demodulation means, connected in circuit intermediate said electronic filter means and said amplifying means. for demodulating the electrical signal furnished by said main light-sensing means.

9. In a method for the measurement of the relative opacity of a gaseous material wherein a modulated light beam of a pre-selected frequency is passed through said gaseous material, and wherein said light beam is reflected back through said gaseous material by a retroreflector positioned across said gaseous material from the source of said modulated light beam, the improvement in said method which comprises the steps of:

periodically and in repetitive cycle, passing said modulated light beam through said gaseous material to be returned therethrough by said retro-reflector;

sensing said returned light beam and generating a first electrical signal representative of the amount of light returned;

storing said first electrical signal as a constant value;

periodically and in repetitive cycle, blocking transmission of said modulated light beam to said gaseous material and reflecting substantially all of said modulated light beam back toward the source of said modulated light beam;

sensing said reflected light beam and generating a second electrical signal representative of the amount of light reflected;

storing said second electrical signal as a constant value; and

electronically subtracting the value of said first electrical signal from the value of said second electrical signal, to thereby obtain a relative opacity value. 

1. An improved transmissometer of the type wherein a modulated light beam of a pre-selected frequency is passed through a gaseous sample, and wherein said light beam is reflected back through said sample by a retro-reflector positioned across said sample from the source of said modulated light beam, said improved transmissometer comprising, in combination: a main light-sensing means, positioned out of the path of travel of said modulated light beam, for generating an electrical signal representative of the amount of light falling thereon; a movable chopping means, having at least three distinct segments, for sequentially and in repeating cycle, moving said three segments into the path of said modulated light beam, said segments including: a cut-out portion to allow passage of said modulated light beam through said sample; a first reflector portion to reflect back substantially all of said modulated light beam; and a second reflector portion to reflect back a pre-selected percentage of said modulated light beam; means, positioned in the path of travel of said modulated light beam in advance of said chopping means, for passing said modulated light beam toward said chopping means and directing any light from said retro-reflector or said first or second reflector portions onto said main light-sensing means; first electronic means for receiving and presenting as a constant output signal for each cycle of said chopping means, the electrical signal representative of the light reflected by said first reflector portion; second electronic means for receiving and presenting as a constant output signal for each cycle of said chopping means, the electrical signal representative of the light reflected by said retro-reflector; electronic subtraction means for subtracting said electrical signal representative of the light reflected by said retroreflector from said electrical signal representative of the light reflected by said first reflector portion; and sequencing means, connected to said first and second electronic means, for generating an individual electrical signal indicating the presence at said main light-sensing means of light rEflected by said retro-reflector, said first reflector portion or said second reflector portion.
 2. The transmissometer of claim 1 which further includes: timing means, connected to said second electronic means, for conditioning said second electronic means to receive the electrical signal representing the light reflected by said retro-reflector, said first reflector portion and said second reflector portion as individual, independent signals in a pre-selected, repeating time based cycle.
 3. The transmissometer of claim 1 which further includes: display means, connected to said electronic subtraction means, for visually displaying the difference signal generated by said subtraction means.
 4. The transmissometer of claim 1 which further includes: amplifier means, connected to said main light-sensing means and said first and second electronic means, for amplifying the electrical signal from said main light-sensing means.
 5. The transmissometer of claim 4 which further includes means, connected to the output of said first electronic means and to said amplifier means, for comparing the output signal value of said first electronic means with a pre-selected signal value and for generating a correctional signal to vary the gain factor of said amplifier means if said output signal of said first electronic means differs from said pre-selected signal value.
 6. The transmissometer of claim 4 which further includes: a current-to-voltage converter, connected in circuit intermediate said amplifier means and said main light-sensing means, for converting an electrical current from said main light-sensing means to an equivalent electrical voltage.
 7. The transmissometer of claim 6 which further includes: electronic filter means, connected in circuit intermediate said current-to-voltage converter and said amplifying means, for removing all components of the electrical signal furnished by said main light-sensing means which are not at the frequency of modulation of said modulated light beam.
 8. The transmissometer of claim 7 which further includes: electronic demodulation means, connected in circuit intermediate said electronic filter means and said amplifying means, for demodulating the electrical signal furnished by said main light-sensing means.
 9. In a method for the measurement of the relative opacity of a gaseous material wherein a modulated light beam of a pre-selected frequency is passed through said gaseous material, and wherein said light beam is reflected back through said gaseous material by a retro-reflector positioned across said gaseous material from the source of said modulated light beam, the improvement in said method which comprises the steps of: periodically and in repetitive cycle, passing said modulated light beam through said gaseous material to be returned therethrough by said retro-reflector; sensing said returned light beam and generating a first electrical signal representative of the amount of light returned; storing said first electrical signal as a constant value; periodically and in repetitive cycle, blocking transmission of said modulated light beam to said gaseous material and reflecting substantially all of said modulated light beam back toward the source of said modulated light beam; sensing said reflected light beam and generating a second electrical signal representative of the amount of light reflected; storing said second electrical signal as a constant value; and electronically subtracting the value of said first electrical signal from the value of said second electrical signal, to thereby obtain a relative opacity value. 